1. Field of the Invention
The present invention relates to a microwave band-pass filter, and more particularly, to a microwave band-pass filter using a transmission line such as a strip line, a micro-strip line or the like, which has a frequency characteristic of an insertion loss steeply increasing on one outside of the pass-band thereof.
2. Description of the Related Art
Conventionally, there have been used filters in order to take out a desirable signal on a predetermined band, or to reject signals or noise other than a desirable signal. In particular, when a frequency of the desirable signal to be taken out is located between frequencies of signals to be rejected, or when a stop-band is located on both the outsides higher and lower than a pass-band, there is used a band-pass filter.
FIG. 1 shows a conventional microwave band-pass filter of this type, which comprises resonators, each resonator composed of a half-wavelength strip line having both the open ends. Referring to FIG. 1, between input and output terminals, there are formed between a dielectric substrate 500 and another dielectric substrate (not shown), an input strip line 3, three resonators 5 and an output strip line 4 so that the input strip line 3 is electrically connected through an electromagnetic coupling to the first resonator 5 over a quarter of the wavelength, the first resonator 5 is electrically connected through an electromagnetic coupling to the second resonator 5 over a quarter of the wavelength, the second resonator 5 is electrically connected through an electromagnetic coupling to the third resonator 5 over a quarter of the wavelength, and the third resonator 5 is electrically connected through an electromagnetic coupling to the output strip line 4 over a quarter of the wavelength.
In the conventional microwave band-pass filter shown in FIG. 1, when the number of stages of the resonators 5 is increased, the insertion loss on the band on the outside of the pass-band can be steeply increased. However, each of the resonators 5 has a relatively low unloaded Q such as about several hundreds at about 10 GHz. Therefore, when the number of stages thereof is increased, the microwave band-pass filter is increased in size, and also the insertion loss in the pass-band increases. Accordingly, it is extremely difficult to obtain a microwave band-pass filter having a relatively low insertion loss on the pass-band and having a frequency characteristic of the insertion loss steeply increasing on the outside of the pass-band.